The invention relates to an improved technique for the dehydrogenation of hydrocarbons, particularly light hydrocarbons. In preferred embodiments, the invention provides improved techniques for producing isobutylenes through the dehydrogenation of butanes, and propylene through the dehydrogenation of propane; particularly such techniques which minimize metal carburization and associated coke formation during dehydrogenation.
Dehydrogenation processes are of particular interest to the petroleum industry because light hydrocarbons such as butane are low-value by-products from refining operations. Butane can be converted to butylenes through dehydrogenation, which can then be used to produce MTBE.
Conventionally, butylenes, including isobutylene, have been obtained as a by-product from refinery processes such as catalytic or thermal cracking units. However, the demand for isobutylene has so far exceeded the production from such refining operations. Therefore, various alternative processes have been developed to provide isobutylene.
One type of process is the non-catalytic thermal dehydrogenation of organic compounds, e.g., the conversion of butane to butene. However, the effective use of such methods is limited due to the extensive and undesirable side reactions which occur.
Various catalytic processes have been developed in order to minimize side reaction activity and improve conversion and selectivity to desired products. Traditional catalytic dehydrogenation processes include the Air Products Catofin.TM. process, the Universal Oil Products (UOP) Oleflex.TM. process and the Phillips Star.TM. process.
The Air Products Catofin.TM. process allows for the dehydrogenation of butane to form butylene in the presence of a catalyst containing a chromic oxide supported on alumina in an adiabatic reactor. See, e.g., European Patent Application 192,059 and UK Patent Application GB 2,162,082, the contents of which are hereby incorporated by reference.
The UOP Oleflex.TM. process allows for the dehydrogenation of propane to form propylene and of (iso)butane to form (iso)butylene in the presence of a catalyst containing platinum supported on alumina in a moving bed reactor. The moving bed reactor allows continuous catalyst regeneration under the more severe conditions of lower alkane dehydrogenation. The catalyst flows fully from the reactors to the regeneration zone and is then recycled to the reactor. See, e.g., U.S. Pat. Nos. 3,584,060; 3,878,131; 4,438,288; 4,595,673; 4,716,143; 4,786,265; and 4,827,072, the contents of which are hereby incorporated by reference.
The Phillips Star.TM. process allows for the dehydrogenation of butane to form butylene in the presence of a promoted platinum catalyst supported on a zinc-alumina spinel. The catalyst is supported in tubular catalyst beds located within furnaces to provide the endothermic heat of reaction. This arrangement allows for operation under isothermal conditions. Catalysts are regenerated by oxidation in air. See, e.g., U.S. Pat. Nos. 4,167,532; 4,902,849; and 4,926,005, the contents of which are hereby incorporated by reference.
Since dehydrogenation of hydrocarbons is an endothermic reaction and conversion levels are limited by chemical equilibrium, it is desirable to operate at high temperatures and low pressures. High temperatures and low pressures shift the equilibria favorably toward dehydrogenated products. However, conventional dehydrogenation catalysts suffer rapid deactivation by coking under these severe conditions. In particular, it has been found that slow accumulation of carbon deposits reduces the dehydrogenation activity of conventional dehydrogenation catalysts. Thus, conventional carbon burn-off cycles are typically used to regenerate the catalyst system after sufficient accumulation of carbon on the catalyst. In addition, sulfur compounds and hydrogen are usually introduced to the reactor feed in order to prevent carbon build-up in the reactor and catalyst bed.
Industry reports suggest that design inadequacies still exist with commercial scale dehydrogenation processes. For example, as recently reported in Platt's International Petrochemical Report (October 1993), those familiar with the UOP Oleflex.TM. process say that there is a design flaw which causes a coking problem with the heat exchangers after about a year of operation.